Height and shape for a raptor perch?

I'm looking into adding some raptor perches or nesting platforms near a community garden to address gophers or ground squirrels (not sure which).  Red tailed hawks are our primary raptor.  I've seen one publication that suggested 15' from the ground to an 18" diameter disk of wood is a good design.

Anyone have more input?  Is that height necessary or is it on the low side?  Is a platform best or a crosswise stick for them to sit on?  Would they nest on a platform or should I only worry about perching and let them find a nest in the nearby 5000 acres of forest?

The garden is in a 50 acre field if that matters.

Thanks!

I'm also interested in this subject.

The mice ate all my pea seed this year :( , so I'm trying to design in more predator attracting features around my growing area to keep the vole population down a bit. I'm thinking of making a stoat house (need to do a bit more research on that) and a buzzard perch.
I know the buzzards like to perch on fence posts (we often see the tourists go away happy that they have seen a eagle, bless!) so I think just a broad post top will be OK. I'll probably have to splint a few poles together to get a reasonable height, but I'm not sure what reasonable is...

I found this site
https://tommy51.tripod.com/perch.html

Thank you John, that is very helpful.
I'll probably make mine a bit shorter - I'm aiming to protect quite a small area. Well I'll maybe make two since i have two areas a little way apart.

I'll have to get some pics of what I did.  I cut two cedar trees and drove them over to the garden and "planted" them.  I cut most branches off of the trees but left some stubs up near the top.  The taller one was 30 feet long. Haven't seen a raptor perched on them yet...

Mike, how do you plant a 30ft tree?
Are there Raptors in your area?

We dug a 3-4' deep hole, carried the log/tree/chunk over to it, had one person hold the butt end down with a foot while 3 of us lifted the other end up.  We "walked" forward with our hands until it was vertical enough to fall into the hole.

The really fun part was driving them to the garden with my little pickup truck...

How did you find your perch working Mike? I've finally got round to making mine as well. I should have checked this thread before starting since I didn't include the underneath supports for the perches.
We had a 'problem is the solution' moment recently. Someone locally was moving away and offered us their old wardrobes. We didn't really need them, but when it came to them saying they would have to burn them (solid wood!) there was no real choice. So to find a place to put them we had to clear out the Byre, which has become a bit of a dumping ground. Another lot of really scrappy wood was put in there with no time to sort out nice stuff (like teak from an old boat) from rubbish (rotten or too many nails to be useful). So in sorting out the old wood I managed to get out a very long bit of timber that I think was an undersized floor joist. It has nails every 6 inches still sticking out, so is of little use for building wok now, but excellent for a tall raptor perch support!
I wasn't sure whether the buzzards would like a more round perch, so I actually used two different sections for the perchs - one bit of square cut batten wood and one broken axe handle. I screwed them to one end of the floor joist.



I then made sure that the fence post I was using to support the perch post was as vertical as I could get it. I had put one in a while ago, but it was a bit wonky so I repositioned it.



Fiinally I screwed the upright to the fence post. A little tricky by myself - I really could have done with a hand with this, but I managed to lift it off the ground and screwed in the top fixing first. Result - the highest place in the area for a hunting bird to sit (just to the right of my shed):



I'm a bit worried that it won't be up to the job of keeping vertical! In a moderate wind it was swaying somewhat, and I may put in some guyropes if I can work out how to anchor them firmly and position them so they aren't in my way. I may be able to use them to grow more climbers up near my shed.

I believe I can say that they worked!  Robins put in the most hours on the perches but on one occasion I was at the garden and a kestrel landed on one and stayed for a while.  

A raptor perch should be 15-20 feet tall and have a 18-inch diameter disk of wood as a perching surface. While a platform can be used for nesting, a crosswise stick is sufficient for perching. The height is important for providing the raptor with a clear view of its surroundings and enabling it to hunt effectively. For a 50-acre field, multiple perches would be beneficial to provide the raptor with more options and coverage.

There's a couple of families of native falcons (karearea) that nest in the cliffs either side of our community garden that are regular visitors.

They don't  seem to be too fussy about perches and are happy to sit on our bean trellises and assortment of poles and garden shed roofs.

The karearea is endangered so we are very fortunate to have them help keep down the population of mice and rabbits that have taken up residence in our community garden.

The only downside is that our chickens can no longer be allowed to free range under our fruit trees.